Braking shield for a spacecraft, and a satellite fitted therewith

ABSTRACT

The invention relates to a braking and heat protection shield (1) for a spacecraft (5), said shield being situated at the front of the spacecraft to oppose a flow of gas striking the spacecraft, and including at least one gap (3a, . . . , 3d) shaped to present a through section to the flow that increases with increasing angle of incidence of the spacecraft, to create an aerodynamic force couple tending to return the spacecraft towards a position of lesser incidence.

The present invention relates to a braking shield for a spacecraft, andmore precisely, but not exclusively, a shield that is designed to slowdown an earth observation satellite by friction with the layers of theatmosphere at low altitude (typically 120 km to 140 km).

To put a satellite into low orbit, it is possible to use a speciallyadapted launcher.

Nevertheless, it is sometimes economically more advantageous to takeadvantage, whenever possible, of space that remains available in thenosecone of a standard launcher adapted to put a payload into highorbit, and then to actuate propulsion means with which the satellite isfitted to send it into low orbit.

To reduce the quantity of propellant on board the satellite, proposalshave been made to place a braking drag at the back of the satellite forthe purpose of slowing down the satellite by friction with the lowaltitude layers of the atmosphere and consequently of reducing theapogee altitude of the satellite's orbit. The satellite propulsion meansare then no longer actuated for performing braking directly, but for thepurpose of sending the satellite into the low altitude layers of theatmosphere where it loses energy by friction and then, after braking, tocause it to leave those layers of the atmosphere in order to reach itsfinal orbit.

The braking drag is placed at the back of the satellite so as to imparta stable attitude to the satellite, i.e. an angle of incidence that isconstant relative to the flow of gas striking it.

Braking shields are also known for atmospheric re-entry of craft thathave gone beyond the atmosphere, with such shields being in the form ofnosecone-shaped shells (to impart a stable attitude to the spacecraft)placed at the front thereof, and also providing heat protection to thespacecraft fitted therewith by deflecting the flow of gases.

It is desirable to fit satellites with shields that are placed in frontso as to combine the functions of providing both braking and heatprotection. Nevertheless, because such shields are nosecone-shaped, theysometimes take up more room in a longitudinal direction than iscompatible with the space available in the nosecone of the launcher,particularly when advantage is being taken of some other satellite beingput into a high orbit by a standard launcher, as mentioned above.

The present invention seeks to propose a novel braking shield for aspacecraft, in particular an observation satellite, the shield being ofsize compatible with the space available under the nosecone of alauncher, and serving to provide the spacecraft simultaneously withattitude stability and with heat protection.

The shield of the invention is characterized in that it includes atleast one gap shaped in such a manner as to provide a through section tothe flow of gas striking the spacecraft, which section increases withincreasing angle of incidence of the spacecraft, for the purpose ofcreating an aerodynamic force couple tending to return the spacecrafttowards a position of lower incidence.

Thus, the invention serves to stabilize the attitude of the satellite inpassive manner, without it being necessary to adopt a nosecone-shapedshell.

In a preferred embodiment of the invention, the shield comprises asuccession of walls having axial symmetry about the longitudinal axis ofthe spacecraft, said walls being coaxial and leaving between them atleast one annular opening of through section offered to the flow thatincreases with increasing angle of incidence of the spacecraft.Preferably, the shield includes a succession of truncated cones disposedcoaxially so as to appear as a solid surface in front view at zeroincidence, and, at non-zero incidence, to appear as a surface havinggaps creating an aerodynamic couple tending to return the spacecrafttowards a position of zero incidence.

The present invention also provides a satellite fitted with a shield asspecified above.

Other characteristics and advantages of the present invention appear onreading the following detailed description of a non-limiting embodimentof the invention, and on examining the accompanying drawing, in which:

FIG. 1 is a front view of a shield of the invention at zero incidence;

FIG. 2 is an axial section through the shield shown in FIG. 1;

FIG. 3 is a view analogous to FIG. 1, at non-zero incidence;

FIG. 4 is an axial section through the shield as shown in FIG. 3; and

FIG. 5 is a diagrammatic side view of a satellite fitted with the shieldof the invention as shown in FIGS. 1 to 4.

The shield 1 shown in the figures has walls referenced 2a, . . . , 2ewhich are in the form of truncated cones in the example described andwhich are axially symmetrical about an axis of symmetry X that is normalto the plane of FIG. 1 and that is contained in the section plane ofFIGS. 2 and 4. The walls 2a, . . . , 2e are inclined relative to saidaxis X, and they converge forwards.

The walls 2a, . . . , 2e are disposed coaxially and between them theyleave annular openings 3a, . . . , 3d. The dimensions and theorganization of the walls 2a, . . . , 2e are selected so that when seenin front view and at zero incidence, i.e. when the axis of symmetry Xcoincides with the direction E of gas flow striking the shield 1, saidshield presents a solid surface (FIG. 1), whereas when the angle ofincidence α is not zero, it presents a surface with gaps (FIG. 3) so asto create an aerodynamic force couple tending to return the shield 1towards a position of zero incidence. The outside diameter of the shield1 is selected so that it presents sufficient braking area and offersheat protection, by deflecting the flow of gases away from the body 5 ofthe satellite towards external elements (solar panels, sensors, . . . )of the satellite, placed on the body 5 of the satellite downstream fromthe shield 1 relative to the gas flow.

The radially inner edges 2a', 2b', 2c', 2d' of the walls 2a, 2b, 2c, and2d coincide in projection along the direction of the axis X,respectively with the radially outer edges 2b", 2c", 2d", 2e" of thewalls 2b, 2c, 2d, and 2e. The walls 2a, . . . , 2e extend axially overthe same height, with the radially inner edges (or front edges) 2a', . .. , 2e' being situated in a first plane P₁ perpendicular to the axis X,and with the radially outer edges 2a", . . . , 2e" (or rear edges) beingsituated in a second plane P₂ perpendicular to the axis X, and situateddownstream therefrom.

The walls 2a, . . . , 2e are assembled together by any appropriatestructure known to the person skilled in the art, e.g. by radialstiffeners 4, with there being eighteen of them in the exampledescribed, extending radially and uniformly spaced apart around the axisX. In the example described, the stiffeners 4 are in the form of pairsof parallel rods extending in said above-mentioned first and secondplanes, and forming spacers for the walls 2a, . . . , 2e. To furtherimprove stiffness, it is possible to use additional structural elementssuch as bars connecting the wall 2a (or the ends of some of thestiffeners) to the sides of the body of the satellite.

The shield shown in FIGS. 1 to 4 is fitted to the front of a satellitewhose body 5 is elongate along a longitudinal axis coinciding with theaxis X. The shield 1 is fixed to the body 5 of the satellite by anyappropriate means known to the person skilled in the art, e.g. byholding struts 8 connecting the rods of the stiffeners 4 situated in theplane P₂ to the front face 7 of the body 5. In a variant, the holdingstruts may also connect the wall 22 to the body of the satellite.

The radially inner edge 2e' of the wall 2e defines a central opening 6of the shield 1, and its diameter is preferably selected to be equal tothe diameter of the front face 7 of the body 5 of the satellite, asshown, such that at a zero angle of incidence, the plane front face 7 ofthe body 5 of the satellite opposes the flow passing through the opening6. At a non-zero angle of incidence, the wall 2e and the front face 7provide a non-zero through section to the flow, between the holdingstruts 8.

In the example described, the front face 7 of the body 5 of thesatellite has a diameter equal to 800 mm, the shield 1 has an outerdiameter (measured from the edge 2a") equal to 2000 mm, and an innerdiameter (measured from the edge 2e') equal to 800 mm. The radiallyinner edges 2a', . . . , 2e' of the walls 2a, . . . , 2e are spacedapart at radial intervals of 120 mm. Each wall 2a, . . . , 2e has aheight of 200 mm and the same half-angle at the apex, which is equal to31° in the example described. The position of the center of thrust movesalong the axis of symmetry X and lies between 200 and 300 mm behind theinterface between the shield and the body of the satellite, depending onwhere the angle of incidence α between the axis of symmetry X and therelative speed between the satellite and the atmosphere lies in therange 5° to 31°. In FIG. 5, arrow P designates the direction of theresultant of the thrust forces on the satellite for an angle ofincidence α equal to 30°, which corresponds to FIGS. 3 and 4. The centerof thrust C is situated on the axis of symmetry X behind the center ofgravity G, thereby guaranteeing that the attitude of the satellite isstable.

Finally, the invention makes it possible to have a braking shield ofsmall size in the longitudinal direction, and depending on the positionof the center of gravity of the satellite, it either reduces theinstability of the satellite, thereby making it possible to make do forcontrolling the attitude of the satellite with a system having controlcapacity that is less than that which would be necessary in the absenceof the shield, or else, and preferably, to ensure that the attitude ofthe satellite is stabilized in self-contained and completely passivemanner, as is the case for the example described. The shield 1preferably does not include any moving parts requiring a stowage anddeployment mechanism, thus ensuring that it is particularly reliable.Also, the shield directs the flow of gas striking the satellite radiallyoutwards and provides complete heat protection to external elements onthe body of the satellite (sensors, antennas, solar cells) situatedbehind the shield.

The invention is not limited to the example described, and inparticular, without going beyond the ambit of the present invention, itis possible to modify the number and the shape of the walls constitutingthe shield, e.g. to use a shape that is polygonal rather than beingcircularly symmetrical. The front face of the satellite body may alsopenetrate inside the shield, and indeed project beyond the plane P₁. Thewalls constituting the shield may be folded to reduce bulk during launchand may subsequently be deployed in flight by an appropriate mechanism.

A shield of the invention can be fitted to any type of spacecraft, e.g.a planetary probe that is to make use of atmospheric braking to reach alow orbit around any heavenly body.

I claim:
 1. A braking and heat protection shield (1) for a spacecraft(5), the shield being situated at the front of the spacecraft to opposea flow of gas striking the spacecraft, and being characterized in thatit comprises a succession of coaxial walls (2a, . . . , 2e) having axialsymmetry about the longitudinal axis (X) of the spacecraft, thedimensions and the organization of said walls being selected so that, infront view and at zero incidence, the shield presents a solid surfaceand said walls leave between them annular openings (3a, . . . , 3d) ofthrough section offered to the flow that increases with the increasingangle of incidence of the spacecraft, so as to create an aerodynamicforce couple tending to return the spacecraft to a position of lesserincidence.
 2. A shield according to claim 1, characterized in that saidwalls (2a, . . . , 2e) are inclined relative to said longitudinal axis,and converge forwards.
 3. A shield according to claim 2, characterizedin that the leading edges (2a', . . . , 2e') lie in a common plane (P₁).4. A shield according to claim 2, characterized in that the trailingedges (2a", . . . , 2e") of said walls (2a, . . . , 2e) lie in a commonplane (P₂).
 5. A shield according to claim 1, characterized in that eachof said walls (2a, . . . , 2e) is in the form of a truncated cone.
 6. Ashield according to claim 5, characterized in that said walls all havethe same half-angle at the apex.
 7. A shield according to claim 6,characterized in that the half-angle at the apex of said walls is equalto 31°, their axial extent being 200 mm, and their radial spacing being120 mm.
 8. A satellite fitted with a shield according to claim 1.